Hard diffusion formed reaction coatings

ABSTRACT

A NOVEL PROCESS FOR PRODUCING HARD COATINGS BY DIFFUSING ONE OR MORE REACTIVE ELEMENTS IN MOLTEN LEAD AND REACTING WITH ONE OR MORE HARDENING ELEMENTS OF THE GROUP CARBON, NITROGEN, BORON, AND SILICON PRESENT IN FERROUS PRODUCTS. THE PROCESS OFFERS FLEXIBLITY IN COMPOSITIONAL CONTROL AND COATING PARAMETERS.

United States Patent O 3,795,537 HARD DIFFUSION FORMED REACTION COATINGSRay Joseph Van Thyne, 10148 S. Cook Ave., Oak Lawn, Ill. 60453, and JohnJacob Rausch, Rte. 2, Box 177, Antioch, II]. 60002 No Drawing.Continuation-impart of application Ser. No. 768,187, Oct. 16, 1968, nowPatent No. 3,620,816. This application Nov. 12, 1971, Ser. No. 198,413

Int. Cl. C23c 9/00 US. Cl. 117-114 R 9 Claims ABSTRACT OF THE DISCLOSUREA novel process for producing hard coatings by diffusing one or morereactive elements in molten lead and reacting with one or more hardeningelements of the group carbon, nitrogen, boron, and silicon present inferrous products. The process offers flexibility in compositionalcontrol and coating parameters.

CROSS REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of our copending application Ser. No. 768,187entitled Method of Diffusion Coating Metal Substrates Using Molten Leadas Transport Medium, filed Oct. 16, 1968, now US. Pat. 3,620,816.

BACKGROUND OF THE INVENTION This invention relates to a novel processfor producing hard surface compound layers on formed ferrous products.Lead is used as the medium to transfer a reactive element such aschromium to combine with a hardening element such as carbon present inthe ferrous product. Formation of chromium carbide on steel by packchromizing is a well known art. Such hard compounds may also be appliedas coatings by a variety of methods such as pyrolytic plating andspraying but these processes are not related to the present invention.

In our pending US. patent application, Method of Diffusion Coating MetalSubstrates Using Molten Lead as Transport Medium, Ser. No. 768,187, nowUS. Pat. 3,620,816, we have shown that a lead medium very successfullyproduces corrosion resistant chromized and other surface alloyedimpregnation zones on ferrous materials. Such patent application isdirected to corrosion resistant products with particular emphasis onavoiding carbon reactions which deteriorate corrosion resistance. Wehave now found that a lead medium offers other utility and may be usedto form hard compounds when carbon and other hardening agents arepresent in the ferrous product.

US. Pat. 3,184,331 to Carter discloses a process for diffusion coatingof ferrous materials in molten calcium and other active metal baths.This is directed to corrosion resistance and avoidance of carboncontamination. Calcium is a strong getter for carbon and nitrogen. Forexample, Table II of Carter shows that the carbon content in hischromized coating on type 1070' steel (0.70% C) may be reduced to only12% of the level in the substrate. A lead media behaves quitedifferently from such an active bath and as shown herein is an excellentmedium for producing high carbon, hard coatings. Lead has an exceedinglylow solubility for iron at the temperature of interest; simple ironcontainers may be used and the lead does not corrode the ferrous partsto be coated. The lead may be alloyed with other inert diluents if thedesirable properties of lead are retained.

The object of this invention is to provide a novel process for diffusingat least one reactive metal through a lead bath and forming a hardcompound coating on 3,795,537 Patented Mar. 5, 1974 ferrous articlescontaining one or more elements of the group carbon, nitrogen, boron,and silicon.

DESCRIPTION OF THE INVENTION Our process offers many advantages: (1)absolute uniformity of the hard coating over irregular surfaces, (2)thickness control of the diffusion formed coatings by regulation oftemperature and'time, (3) uniform coating in small orifices, and (4)ease of producing alloyed or mixed compounds since two or more reactiveelements may be readily codiffused through the lead.

The process consists of contacting the ferrous parts with molten leadwhich contains the diffusing reactive element. Contact with air must beavoided or minimized. Various contacting methods may be employedincluding the use of a slurry paint. In the preferred method, theferrous parts, lead, and diffusing reactive elements are sealed in aferrous container and agitated to insure solution of the diffusingelements and to provide relative motion between the ferrous parts foruniform coating. A range of temperatures and times may be used. We havetypically used 1950 F. for 6 hours. For experimental coating of smallparts, We have sealed the constituents in an evacuated tube made of mildsteel or 400 series stainless steel and shaken the tube every 15 minuteswhile at temperature. A perforated spacer separates the tube into twocompartments so that at the end of the run the tube may be inverted andthe lead drained from the parts. In addition, the spacer keeps the partsand the chromium source under the lead during the run; iron and chromiumfloat in molten lead. Only a small amount of residual lead remains whichcan be removed as desired. The lead is reuseable and easily purified.

In a simple example of our invention, C1018 and C1080 steel were reactedwith 3 grams of chromium in 200 grams of lead at 1950 F. for 6 hours.Since chromium is only slightly soluble in lead at 1950 F., all of thechromium does not dissolve. As chromium reacts to form chromium carbide,additional chromium dissolves from the supply. Any source of chromiumsuch as unalloyed metal or ferrochromium may be used. A surface reactionlayer was observed metallographically with both steels. Microhardnessreadings were taken on polished cross-sections using a Leitz Miniloadtester and unless otherwise indicated a load of 15 grams was employed.The reacted C1018 exhibited a hardness of 1380 and 455 DPH at a depthfrom the surface of 7 and 12 microns, respectively (25.4 microns=1 mil).With the availability of a greater carbon level in C1080, the hardcarbide coating is over 1 mil deep. At a depth of 25 microns thehardness was 1500 DPH.

Cast iron was reacted with chromium in lead at 1950 F. for 4 hours andresulted in a 1.3 mil coating. The hardness was 1840 DPH (50 g. load) ata depth of 25 microns. Beneath the coating at a depth of microns thehardness was 295 DPH (50 g. load).

To illustrate the uniformity of coating, a hole inch is diameter by 7inch long was drilled through a specimen prepared from Drill Rod (highcarbon steel). After reacting with chromium in lead at 1950 F. for 6hours, a uniform one mil coating was observed in the hole and over theoutside surfaces. The hardness at a depth of 13 microns halfway throughthe length of the hole was 1500 DPH and the hardness on the outside atthe same depth was 1440 DPH. Thus, the hardness was the same withinexperimental error. A similarly-treated sample with a blind hole 5 inchin diameter by inch deep was found to be uniformly coated including thebottom of the blind hole. 1

Samples of C1018 steel have also been reacted with chromium in a leadbath after first providing commercial carburizing and nitridingtreatments such as cyaniding and Tufftriding. The ferrous materials cancontain carbon, nitrogen, boron, or silicon as potential hardeningelements. The uniform composition of such elements in the ferrousmaterial can be limited since these elements will diffuse outward to thesurface to react with the infusing reactive element from the lead. Thesehardening elements may be present from a prior surface diffusiontreatment.

Molybdenum and columbium have been co-diffused with chromium in lead. Itwill be apparent to those skilled in the art that many reactive elementscan be diffused in lead singly or in combination to form reactioncompounds with one or more elements of the group carbon, nitrogen,boron, or silicon. Such reactive elements include but are not limited tomolybdenum, tungsten, vanadium, columbium, tantalum, titanium,zirconium, hafnium, yttrium, and aluminum.

For greatest utility the reaction compounds should have a minimumhardness of 900 DPH. The hardness can be measured only when the coatingsgrow to sufficient thickness so that a microhardness indentation can bemade on a polished cross-section. However, the compounds haveconsiderable utility when present as thin coatings such that thehardness can not be varified.

Our examples have been directed to ferrous materials but the processcould be employed with other substrates. The hard surfaced materialsproduced by our process will find utility principally in wear resistantapplications but it should be noted that certain of these hard compoundsalso possess other desirable properties such as corrosion resistance.

It will be understood that various modifications and variations may beeffected without departing from the spirit or scope of the novelconcepts of our invention.

We claim as our invention:

1. The process of forming a hard compound coating consisting of a metaland nonmetal reaction product on a ferrous base substrate whichsubstrate contains an element selected from the group consisting ofcarbon, nitrogen and boron and mixtures thereof which comprises thesteps of contacting said substrate with a molten alloy bath consistingprincipally of lead and containing at least one diffusing elementreactive with the group consisting of carbon, nitrogen, boron andmixtures thereof and reacting the surface zone of said substrate withsaid diffusing element to form a hard compound coating, which coating ischaracterized by a surface microhardness of at least 900 diamond pyramidnumerals when the compound coating is grown sufficiently thick that itcan be measured on a cross-section with a diamond pyramid indentor usinga 15 gram load.

2. The process as defined in claim 1 wherein at least one reactivediffusing element is present from the group chromium, molybdenum,tungsten, vanadium, columbium, tantalum, titanium, zirconium, hafnium,yttrium, and aluminum.

3. The process as defined in claim 1 wherein chromium is the reactivediffusing element.

4. The process as defined in claim 1 wherein at least one reactivediffusing elementis present from the group molybdenum, tungsten,vanadium, columbium, tantalum, titanium, zirconium, hafnium, yttrium,andaluminum.

5. The process as defined in claim 1 wherein carbon, nitrogen, ormixtures thereof are present in the ferrous article.

6. The process as defined in claim 1. wherein carbon is present in theferrous article.

7. The process as defined in claim 1 wherein carbon, nitrogen, ormixtures thereof are present in the ferrous article and at least onereactive diffusing element is present from the group chromium,molybdenum, tungsten, vanadium, columbium, tantalum, titanium,zirconium, hafnium, yttrium, and aluminum.

8. The process as defined in claim 1 wherein carbon is present in theferrous article and at least one reactive diffusing element is presentfrom the group chromium, molybdenum, tungsten, vanadium, columbium,tantalum, titanium, zirconium, hafnium, yttrium, and aluminum.

9. The process as defined in claim 1 wherein carbon is present in theferrous article and chromium is the reactive diffusing element.

References Cited UNITED STATES PATENTS 2,864,731 12/1958 Gurinsky et a1.l486.l1 2,926,111 2/1960 Schweitzer et a1. 1486.11 2,929,741 3/1960Steinberg 117l18 X 2,399,848 5/1946 Becker et a1. l1722 2,685,543 8/1954Sindeband 1l7-l18 X 2,685,544 8/1954 Sindeband l4831.5 X 2,685,5458/1954 Sindeband 1483l.5 X 2,910,379 10/1959 Gurinsky 117-1l8 X RALPH S.KENDALL, Primary Examiner U.S. Cl. X.R.

ll71l8, 127, 135.1; l48-6.ll, 15.5

